The present invention relates to seat frame assemblies for motor vehicles and a method for forming the same.
In a typical motor vehicle, the vehicle seats are constructed with a metal seat frame assembly, including a seat back frame and seat frame, and a seat back cushion and seat cushion mounted in overlying relation on the seat back frame and seat frame, respectively. The seat back frame has lower portions thereof operatively connected with rearward portions of a metal seat frame. The seat frame can be of several different configurations, such as the cross-wire type in which a plurality of parallel and/or diagonal metal cross wires are interconnected between support brackets formed into a substantially rectangular configuration, and the pan type in which a shallow rectangular pan is mounted on support brackets. Seat back frames, on the other hand, typically include a pair of spaced vertical support portions defining opposite sides thereof and rigid horizontal support members interconnected therebetween. The seat back frames also may include the parallel/diagonal cross wires.
The connection between the seat back frame and seat frame is usually formed with a reclining mechanism that is capable of permitting the seat back frame to be pivotally movable with respect to the seat frame so as to enable a seat occupant to adjust the angle at which the seat back cushion is reclined with respect to the seat cushion for maximized comfort. Because the reclining mechanism connects the rearward portion of the seat frame with the lower portion of the vertical supports of the seat back frame, it can be appreciated that the lower portion of the spaced vertical supports are particularly subjected to large amounts of mechanical stresses, especially when the seat back is reclined at large angles with respect to the seat, and even more so when an individual occupies the seat.
While it is important to manufacture the vehicle seat frame assembly with sufficient strength, it is equally important that the frame assembly not be made from excessive amounts of metal, otherwise material costs and vehicle weight will be unnecessarily high. Thus, for the most part, the vertical support portions and cross members of the seat back frame are hollow (tubular) in form. To accommodate for the aforementioned mechanical stresses, however, it has been known to manufacture at least one of the spaced vertical supports with a relatively large wall thickness, cross sectional diameter and strength. It is especially important for the lower portion of the heavier vertical support, where the majority of the stress is encountered, to be of superior strength. While it is possible for both vertical support portions to be of the heavier construction (and is actually preferred in a bench seat configuration), sufficient strength for a single occupant seat (such as a bucket seat) is accomplished with only one of the vertical supports manufactured in such fashion. Providing only one of the vertical supports with a heavier construction is advantageous because less metal, without compromising frame strength.
The heavier vertical support portion (or "main vertical support") is also advantageous because it enables the frame to mount a vehicle seat belt. More specifically, in certain vehicles, it may be desirable to mount a seat belt assemblies directly to the seat, as opposed to the vehicle floor. To accomplish this, however, it is necessary to manufacture the seat frame with sufficient rigidity and strength to withstand the inertial forces encountered in a collision. With the heavy vertical support member, a shoulder belt guide can be mounted on an upper end thereof without the potential for buckling of the main vertical support member in a collision. A belt reel can then be mounted at a rearward portion of the seat frame in proximity to the lower portion of the main vertical support member. The belt extends upwardly behind the seat back frame and through the shoulder belt guide forwardly of the seat back frame, and then downwardly until it terminates at an anchor fixed to the rearward side of the seat frame, adjacent the reclining mechanism. A belt clasp is slidingly disposed on a portion of the belt between the belt guide and the anchor. A separate belt member is anchored at a side of the seat frame opposite the reclining mechanism, and terminates at a belt latch fixed thereto. A seat occupant can then easily grasp the belt clasp on the shoulder belt and bring it over his or her shoulder, diagonally across the chest and into latching engagement with the belt latch. With the shoulder belt guide mounted on an upper corner portion of the seat back frame, it can be appreciated that, upon a collision, the main vertical support mounting the shoulder belt guide is once again subjected to a great amount of mechanical stress, especially towards the lower portion thereof.
In producing the main vertical support, manufacturers conventionally employ a stamping technique. Stamping is accomplished by utilizing one or more opposing stamping die members which compress (by impact) a sheet of raw steel therebetween into a desired configuration. To manufacture the main vertical support by stamping, two separate longitudinal sheets of material are individually stamped into a C-shaped transverse cross-section configuration, and then seam welded in facing relation into a tubular configuration.
While it may be desirable to manufacture a main vertical support which is straight, the more usual situation requires the main vertical support to be somewhat bent to accommodate the desired shape of the seat. Thus, after assembly into an elongated tubular configuration, the main vertical support may be bent in a bending mill as required. Alternatively, the individual C-sections may be stamped into a particular shapes that will provide the desired bends when assembled into the final main vertical support.
The stamping process is relatively slow, as it requires raw material to be stamped, then seam welded into tubular elongated form, and then optionally bent. In addition, the process is relatively expensive, as stamping dies and equipment are costly and must be replaced on a frequent basis. Another problem associated with stamping a straight metal sheet is that a significant amount of raw material is often wasted, especially where irregularly shaped frame parts are required. More specifically, where irregular shapes are to be stamped from a sheet of raw metal, there is a significant amount of peripheral material at the side edges of the sheet that cannot be used, irrespective of how closely the stamped shapes are nested on the sheet. In one analysis, it was found that up to 30% of a roll of raw material could not be used due to the irregular shape of the stamped part. It can be appreciated that, at high volumes, the amount of material wasted, and costs associated therewith, is significant in the stamping method. It can also be appreciated that labor, material, and equipment costs associated with manufacturing and assembly of stamped frame members is significant.
In recent years, a hydroforming procedure has been used to replace stamping for certain applications. For example, hydroforming is proposed as a preferred method of forming a vehicle siderail frame assembly in copending U.S. patent application No. 08/314,496. Hydroforming utilizes fluid pressure applied internally to a tubular blank to expand the blank into an irregular shape. More particularly, the tubular blank is placed into a hydroforming mold, and water pressure of up to 10,000 atmospheres, and possibly even greater, is provided interiorly to the blank as opposite ends of the blank are pushed inwardly to obtain material flow. The tubular blank eventually takes the shape of the interior surface of the hydroforming mold. During the hydroforming process, the amount of metal per unit length of tube is varied. More specifically, as the linear exterior circumference is outwardly expanded, metal is replenished from the side portions so that the wall thickness can be maintained within .+-.10% of the original blank. In the areas of greatest expansion, the exterior circumference can be increased by more than 16%, while the wall thickness is maintained within the .+-.10% range.
Strengthening of certain portions of a hydroformed tubular member can be accomplished by expanding those portions more than others. The greater the expansion, the greater the amount of metal per unit length will be provided, and the harder the metal will become.
The inventors of the present invention have contemplated use of hydroforming to manufacture a seat back frame main vertical support. The hydroforming technique is more cost effective than stamping, as a single tubular blank can be formed into a desired elongated integral tubular configuration, without welding or bending. In addition, because metal is stretched during the hydroforming process, it is mechanically strengthened, and less material needs to be used in forming the required diameter tubular structure.
However, as described above, in the hydroforming technique, the wall thickness remains relatively constant, even as the tubular blank is expanded. Thus, while the hydroformed tube can be strengthened in certain areas by expansion, the process is limited in its ability to produce a tubular member that has a greater wall thickness at certain areas, as is desirable in manufacturing a main vertical support for a seat back frame. While the entire main vertical support can be provided with as great a wall thickness as desired throughout, an unnecessarily large amount of material would need to be used toward the upper portions of the main vertical support to accommodate the desired thickness towards the lower portions.
There is thus a need to provide a seat frame assembly which overcomes all of the problems noted above. It is therefore an object of the present invention to meet this need. According to the present invention, there is provided a seat frame assembly for a motor vehicle comprising a seat frame and a seat back frame. The seat frame is constructed and arranged to have a seat cushion mounted thereon in overlying relation. The seat back frame is connected with the seat frame and has a pair of spaced metallic vertical support portions at opposite sides thereof, the seat back frame constructed and arranged to have a seat back cushion mounted thereon. At least one of the vertical support portions are formed from a pair of telescoping tubular blanks including an elongated inner metal blank formed into an integrally formed vertical member of said at least one vertical portion extending from a lower portion of the seat back frame connected with the seat frame to an upper end of the seat back frame and a relatively shorter outer metal blank formed into a sleeve rigidly secured in surrounding relation to the lower portion of the vertical support member. The outer blank and inner blank are formed into the sleeve and the vertical member respectively during a hydroforming process in which the inner blank is expanded so that an outer surface thereof is moved into peripheral contact with an inner surface of the outer blank and the inner blank and the outer blank are expanded into peripheral contact with die surfaces defining the exterior surfaces of the one vertical support portion.
It is a further object of the present invention to provide a method for manufacturing a seat frame assembly for a motor vehicle. The method comprises the steps of forming a seat frame adapted to support a seat cushion mounted thereon; forming a seat back frame having a pair of spaced vertical tubular support portions at opposite sides thereof and adapted to support a seat back cushion mounted thereon, one of the vertical support portions having a generally greater weight and diameter than the other and formed by 1) telescopingly moving a sleeve member over one end portion of a metallic tubular member, 2) placing the metallic tubular member with the sleeve into a cavity of a die mold, the die mold having an interior surface defining a shape of the cavity, 3) providing a fluid internally to the metallic tubular member with sufficient pressure so as to expand the tubular member outwardly so that an exterior surface thereof at the one end portion is moved into peripheral surface engagement with an interior surface of the sleeve, and so that other portions of the tubular member are moved into engagement with the interior surface of the die mold to substantially conform the tubular member to the designed shape of the cavity.
It is understood that the benefits of accomplishing strength at the lower portions of the main vertical support are important irrespective of whether a reclining mechanism connects the seat frame with the seat back frame and whether a belt is attached to the main vertical support, due to the stresses present when an occupant is in the seat.
It is a further object of the present invention to provide an entire seat assembly which incorporates the seat frame assembly manufactured in accordance with the principles of the present invention.